1. Field of the Invention
The invention relates in general to the coating of particulate materials which are so fine and with such high surface to volume ratios that surface phenomena tend to dominate their properties. Particularly, the invention relates to the economical coating of fine particles utilizing chemical vapor deposition procedures in a recirculating fast-fluidized or turbulent fluidized bed reactor under conditions where each particle is coated individually and agglomeration is minimized. Such fine particles may be as small as approximately 0.1 micron in diameter with aspect ratios ranging from approximately 1 to as much as approximately 100. The invention is particularly suited to the treatment of Geldhart's class C particles. The coated particulates find application, for example, as improved starting materials for plasma spray, powder metallurgy, ceramic and composite materials, conductive fillers, reinforcements, and electronic materials. For example, composite products fabricated by utilizing such fine, coated particles in traditional methods of manufacturing, such as cold pressing and hot isostatic pressing, are easily fabricated, have a reduced matrix content and exhibit superior mechanical properties and wear resistance.
2. Description of the Prior Art
Chemical vapor deposition procedures have been utilized for more than a century to apply various coatings to surfaces as well as other applications. The process involves the atom by atom accumulation of a gaseous compound onto the surface of a heated substrate. The gaseous compound undergoes a thermal or chemical decomposition as it contacts the heated substrate, resulting in deposition of the material onto the substrate. The reaction rate and consequent buildup of the compound on the substrate is governed by a number of factors, such as the characteristics of the gaseous compound, its concentration, the flow rate of the gas, its temperature and pressure, the characteristics to the substrate, its geometry, and its temperature. The rate and amount of coating can be controlled by varying these parameters. For example, the degree of adherence of the substrate may be controlled by varying the temperature of the substrate. The reactor chamber will typically have a heating jacket or other means of heating the reaction zone where the chemical deposition takes place.
Fluidized beds operating in the fast-fluidized and turbulent fluidization flow regimes had been proposed for various purposes, but not in the context of chemical vapor deposition operations, and particularly not with reference to the coating of very fine particulate materials which fall into Geldhart's class C, which are highly cohesive and difficult to fluidize.
A composite material can usually be defined by three entities, the matrix, the reinforcement, and the interface. The matrix of the composite is the continuous phase, interspersed with particulate reinforcement usually in the form of fibers, whiskers, or granular powders. In cermet and metal matrix composites using particulate as a reinforcement, a uniform dispersion of particles throughout the composite and a strong adhesion of these particles to the matrix are of primary concern. Grain size must be controlled to secure the desired properties. Various expedients such as the addition of grain growth inhibitors, had been proposed to control grain size. Difficulties had been experienced in maintaining the matrix between the particulate material so that the particles did not directly contact one another. In an attempt to prevent the particles from touching one another in the composite, previous expedients often involved the use of excessive matrix material which degraded the physical characteristics of the resulting composites. In certain previous expedients, the powder and the binder were physically blended in an attempt to produce a homogeneous distribution. However, the uniformity of the dispersion and the quality of the resulting product was limited by the non-uniform distribution of the matrix and reinforcement during fabrication. Such segregation was an inevitable result of settling and unequal distribution of the admixture.
Various expedients such as liquid solutions, electroless plating, and precipitation methods had been proposed for coating relatively small particles, however such expedients had not proven to be entirely successful for economically and effectively coating very fine particles without agglomeration, and particularly cohesive particles.
These and other difficulties of the prior art have been overcome according to the present invention.